1. Field of the Invention
The invention concerns a method for the manufacture of a superconductor with a layer of the A-15 phase of the system Nb--Al or Nb--Al--Ge, respectively, as well as apparatus for carrying out this method.
2. Description of the Prior Art
The A-15 phases of the systems Nb--Al and Nb--Al--Ge become superconducting at relatively high temperatures and lose this state only in very strong magnetic fields. In the system Nb--Al the A-15 phase has the composition Nb.sub.3 Al or a composition which deviates only slightly therefrom. For Nb.sub.3 Al, the maximum transition temperatures of T.sub.c = 18.8.degree. K as well as upper critical magnetic fields B.sub.c2 of approximately 30 Tesla at 4.2.degree. K have been given. For the A-15 phase of the system Nb--Al--Ge with a niobium content of about 75 atomic precent in the range of compositions of about Nb.sub.3 (Al.sub.1.sub.-x Ge.sub.x) with 0&lt;x.ltoreq.0.25, maximum transition temperatures of T.sub.c =20.7.degree. K and upper critical magnetic fields B.sub.c2 of about 40 Tesla at 4.2.degree. K have been reported (see Zeitschrift fuer Naturforschung 25a (1970), pages 1659 to 1669). These properties of the A-15 phases of the systems Nb--Al and Nb--Al--Ge are not exceeded by any superconductor that has become known to date. The A-15 phases mentioned are therefore of great technical interest, for instance, for application in superconducting magnet coils.
However, the technical application of these materials has not been successful heretofore because of various obstacles. On the one hand, the critical current densities reached so far are relatively low. Thus, melted or powder-metallurgically prepared samples of the A-15 phases mentioned, for instance, have critical current densities j.sub.c of only about 10.sup.3 amps/cm.sup.2 in a magnetic field of 5 Tesla and at a temperature of 4.2.degree. K. Only by three methods has it been possible to date to produce superconducting layers with values of the critical current density, which are of technical interest, of 10.sup.4 amps/cm.sup.2 or more. This includes two methods for the manufacture of superconductors with layers of the A-15 phase of Nb--Al--Ge and a method for the manufacture of a superconductor with a layer of the A-15 phase of Nb--Al. In the two first-mentioned methods, an Nb--Al--Ge layer is in each case applied to a carrier by cathode sputtering or by condensation in a high vacuum (see J. Vac. Sci.Technol. 7 (1970), page 127 ff. and J. Appl. Phys. 43 (1972), page 2407 ff.). In the last-mentioned method (Metallurgical Transactions 3, (1972), pages 1177 to 1181), an aluminized niobium foil having an aluminum diffusion seam is annealed for 8 to 15 seconds at a temperature above 1870.degree. C, to prepare a superconductor with an Nb.sub.3 Al layer. The aluminum was applied here to the niobium foil by melting an aluminum wire over the foil or by vapor deposition or by electrolytic precipitation from a hot aluminum salt bath of about 300.degree. to 500.degree. C. To form the aluminum diffusion seam, the coated foil was heated to a temperature of between 800.degree. and 900.degree. C. Although the methods mentioned furnish A-15 layers with relatively high critical current densities, they do not yet appear to be suitable for the manufacture of technically usable superconductors with such layers. First, each of these methods requires considerable apparatus means, and in addition it is also most doubtful whether the methods, by which up to now only short wire samples have been prepared, can be modified for making long superconducting wires or ribbons, such as are required for winding superconducting coils.
A further difficulty, which so far has been in the way of a technical application of the A-15 phases mentioned, is that these phases are very brittle and can be deformed elastically only to a slight degree. In technically usable superconductors, therefore, the A-15 layers must be only a few .mu.m thick and should lie as close to the neutral fiber of the conductor as possible. With the prior art expensive methods, the manufacture of long conductors with such thin layers would seem to present considerable difficulties. Although methods are known for manufacturing superconductors with layers of A-15 phases of other systems, particularly with layers of Nb.sub.3 Sn and V.sub.3 Ga, which fulfill these requirements, such as the deposition of Nb.sub.3 Sn on a suitable carrier from the gaseous phase or the diffusion of tin or gallium into niobium or vanadium wires, respectively, from copper-tin or copper-gallium sheaths which surround these wires, it has not been possible so far, to apply either method in a satisfactory manner for the manufacture of superconductors with Nb.sub.3 Al or Nb.sub.3 (Al,Ge) layers.
It is an object of the invention to make possible the manufacture of superconductors with a layer of the A-15 phase of the system Nb--Al or Nb--Al--Ge as simply as possible, where at the same time the highest possible transition temperatures and critical current densities which are usable for technical applications are to be achieved. In particular, the continuous manufacture of long wires or ribbons with thin layers of the A-15 phases mentioned should also be possible.